First thing I did after making it 
That was the initial idea (Realtime Midi Processing - Is It Possible)
However I don’t think its possible with the current API, I can’t find any functions that would allow this, if you could then you could use this tool to control VSTi’s as well.
If you want to change recorded pattern data then there are already tools out there that will do this, this one for example: Transposer | Renoise
Cheers